Chuan Sheng Liu

Stimulated coherent Smith-Purcell radiation from a metallic grating

A relativistic electron beam with velocity /spl nu//sub b/z passing over a metallic grating, with deep periodic depressions of wavenumber k/sub 0/z, excites a slow wave (/spl omega/, k/sub z/) via Cerenkov interaction, /spl omega/=(k/sub z/+k/sub 0/)/spl nu//sub b/. The beam bunches induce currents in the fins of the grating that act as a phased radiating dipole array, thereby generating coherent radiation at a wavelength /spl lambda/=(2/spl pi//k/sub 0/l)(c//spl nu//sub b/-sin/spl theta/) where l is an integer and /spl theta/ is the angle of emission measured from the normal of the grating. The current threshold for the instability and radiation power scaling with beam current are discussed.

Excitation of surface plasma waves over metallic surfaces by lasers and electron beams

A laser incident on a metal film (deposited on a glass substrate) from the glass side at a specific angle of incidence, excites a surface plasma wave (SPW) at the metal-free space interface. The ratio of the SPW field to the laser field increases with the laser spot size b attaining a value much greater than one at b>exp(2w/spl alpha//c) where a is the film thickness and /spl omega/ is the laser frequency. The SPW (/spl omega/, k/sub z/,) can also he excited by a relativistic electron beam, propagating parallel to the interface in the free space region, via Cerenkov interaction when beam energy /spl epsiv/b=(|/spl epsiv/|-1)mc/sup 2/ where /spl epsiv/ is the effective metal permittivity, and mc/sup 2/ is the electron rest mass energy. When the surface has a ripple of wave number k/sub 0/, the SPW (/spl omega/, k/sub zz/) can be excited at lower beam energy via sideband coupling, /spl omega/=(k/sub zz/+k/sub 0/)v/sub b/ where v/sub b/z/spl circ/ is the beam velocity. In both cases, however, the positioning of the beam in the close proximity of the interface is required. The scheme is useful for the generation of wavelengths longer than 1 /spl mu/m.

Diffraction-limited laser excitation of a surface plasma wave and its scattering on a rippled metallic surface

A theory of excitation, reflection, and scattering of a surface plasma wave (SPW) over a metallic surface is presented by modeling a localized surface ripple by an electron density perturbation. The level of a laser excited SPW is significantly lowered by the diffraction effects. When an SPW is incident on a bump or a slot, it undergoes specular reflection. It also undergoes elastic scattering when encountering a surface defect, as observed in recent experiments.

Plasma effects in a free electron laser

The introduction of a plasma and a strong guide magnetic field in a free electron laser (FEL) slows down the phase velocity of radiation, significantly reducing the requirements on beam energy for generating frequencies below the electron-cyclotron frequency ( omega /sub 1/ or approximately=17 kA. >

Tunable terahertz radiation from a tunnel ionized magnetized plasma cylinder

A scheme of producing tunable terahertz radiation using a short pulse laser to tunnel ionize a gas jet immersed in a magnetic field is examined. The free electrons born inside the laser pulse retain a finite transverse drift after the passage of the pulse, setting themselves in transverse oscillations at frequencies ωR,ωL=[(2ωp2+ωc2)1/2±ωc]/2, shifted from that without magnetic field at ωp/2, where ωp is the electron plasma frequency and ωc is the cyclotron frequency. The oscillating electron cylinder emits coherent terahertz radiation with ambient magnetic field providing the frequency tunability. The presence of an axial density ripple controls the angular orientation of the emitted radiation.

A slow wave free-electron laser

A simple calculation of a free-electron laser in the Compton regime that uses a dielectric-lined waveguide is presented. The introduction of a dielectric lining in a free-electron laser considerably reduces the requirements on beam voltage for generating a given frequency omega /sub 1/=k/sub 0/c/(1 - v/sub b/ eta /c), where k/sub 0/ is the wiggler wave period eta is the effective index of refraction (1 >

A surface plasmon laser

We examine the possibility of stimulated emission of a surface plasma wave (SPW) on a metal-vacuum interface by electron-hole recombination in a forward biased p-n junction located near the interface. We consider a thin layer of n-type semiconductor sandwiched between a metal and a p-type semiconductor, and the p-n junction is forward biased. The mode structure of the SPW, propagating along the metal surface, extends up to the p-n junction, where it induces electron-hole recombination and gets amplified. The optical gain of the SPW laser can be made comparable to that of a diode laser by reducing the thickness of the sandwich layer.

Explosive instability in a backward wave oscillator

A plasma filled backward wave oscillator supports Trivelpiece-Gould (TG) and TM modes. The former can be driven unstable by a relativistic electron beam via Cerenkov interaction or by the plasma return current as two stream instability. This unstable TG mode can parametrically couple to a TM mode via a negative energy beam mode giving rise to an explosive instability. >

Multi-photon pumping in a large band gap semiconductor laser

Population inversion in a large band gap semiconductor, e.g., ZnSe or ZnS, could be achieved by irradiating it with a picosecond Nd: glass laser of power density exceeding 30 GW/cm2. The laser field induces valence band electrons to tunnel to the conduction band, creating electron-hole plasma. The electron-hole recombination produces coherent radiation at ultraviolet wavelength. Nonlocal effects have a significant influence over the gain.

Electron acceleration by surface plasma waves in double metal surface structure

Two parallel metal sheets, separated by a vacuum region, support a surface plasma wave whose amplitude is maximum on the two parallel interfaces and minimum in the middle. This mode can be excited by a laser using a glass prism. An electron beam launched into the middle region experiences a longitudinal ponderomotive force due to the surface plasma wave and gets accelerated to velocities of the order of phase velocity of the surface wave. The scheme is viable to achieve beams of tens of keV energy. In the case of a surface plasma wave excited on a single metal-vacuum interface, the field gradient normal to the interface pushes the electrons away from the high field region, limiting the acceleration process. The acceleration energy thus achieved is in agreement with the experimental observations.